Digital still cameras, digital video cameras or electronic devices with imaging function (e.g., mobile phones) are equipped with an actuator for positioning a focusing lens. The actuator may be a stepping motor type, a piezoelectric type, a voice coil motor (VCM) type or the like.
The VCM may generate a driving force in a linear direction depending on a direction of current flowing through its coil. For example, when an H bridge circuit is connected to the VCM, a direction of a coil current can be reversed. With such configuration, a driving force in the positive and negative directions may be obtained.
Such an actuator may employ an open loop type or a feedback type (i.e., closed loop type). The open loop type is configured to supply a driving current to the VCM according to a position command value indicating a target position. The feedback type is configured to generate a position detection value indicating a position of a mover of VCM using a hall sensor or the like and drive the VCM such that the position detection value matches a position command value.
In an auto-focusing actuator, when a lens is vibrated after being focused by an auto-focus function, an image deteriorates and/or includes noise. Therefore, there is a need to suppress vibration after moving the lens.
In an actuator of the open loop type, a resonant frequency component is removed from a VCM driving signal and vibration is suppressed by driving the VCM with a driving signal containing no resonant frequency component.
The present inventor has studied feedback type actuators and has come to recognize the following problem. In this actuator, a position command value from a microprocessor is changed step by step. Accordingly, when a VCM driving circuit drives a VCM with a driving signal having the same waveform as the position command value, the driving signal contains a harmonic component which causes vibration of a mover after the mover is moved. Such vibration of the mover due to the harmonic component has not yet been recognized by those skilled in the art but recognized by the present inventor.